In order to investigate the association between uterine tone elevation and fetal heart rate (FHR) abnormalities following labor analgesia - mainly with combined spinal-epidural (CSE) technique - a prospective double-blinded randomized study was conducted with seventy seven parturients who requested labor analgesia. Study group (41 cases) received CSE with sufentanil and bupivacaine and control group (36 cases) received epidural analgesia with the same drugs. Intra-uterine pressure was monitored with intra-amniotic pressure device and FHR with external transducer, both for at least 15 minutes before and 30 minutes after analgesia induction. The primary outcomes were the occurrence of an elevation of 10mmHg or more on uterine tone compared to the values before analgesia and the presence of prolonged fetal heart rate decelerations or fetal bradycardia. Maternal pain scores, blood pressure and use of oxytocin were also computed. A significant association was noticed between elevation of uterine tone and fetal heart rate abnormalities with combined spinal-epidural analgesia, at the first 15 minutes of administration. Uterine tone elevation was observed in 17 out of 41 CSE subjects and only 6 out of 36 controls (p=0.02). Fetal heart rate abnormalities were seen in 11 out of 17 cases that had hypertonus with combined analgesia and in only one of the 6 epidural patients (p<0.001). Logistic regression analysis revealed the mode of analgesia as the independent factor for the elevation of uterine tone, even with oxytocin use as a covariate. It also pointed out the uterine tone elevation as the only independent factor related to the development of fetal heart rate abnormalities, even with maternal hipotension as a covariate. A correlation was found between the fast onset pain relief provided by CSE analgesia and the estimated probability of uterine tone elevation and simultaneous fetal heart rate abnormalities. The present results strengthen the hypothesis that CSE analgesia can lead to a transient increase in uterine tone, leading to fetal bradycardia

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